The invention relates to data synchronization between two or more synchronization devices, particularly to limiting the size of synchronization messages during a synchronization session. Data synchronization is an operation in which a correspondence between at least two data collections is created in such a way that after the synchronization the units of the data collections substantially correspond to each other.
Data of portable terminals, such as portable computers, PDA (Personal Digital Assistant) devices, mobile stations or pagers, can be synchronized with network applications, desktop computer applications or other databases in a telecommunications system. Data of calendar and e-mail applications, in particular, are synchronized. Synchronization has previously been based on different proprietary protocols which are not compatible with each other. This restricts the use of terminal or data types and often causes problems to the user. In mobile communication, in particular, it is important to acquire and update data irrespective of the terminal or application used.
The SyncML (Synchronization Markup Language), which is based on the XML (Extensible Markup Language) has been provided for improved synchronization of application data. The SyncML synchronization protocol using messages in the SyncML format (SyncML messages) allows synchronization of data in any application between any networked terminals. For example, a calendar entry in a mobile station is automatically synchronized with the network calendar used by a company secretary.
FIG. 1 shows an example of synchronization where a mobile station MS functions as the SyncML client terminal and a network server S functions as the SyncML server. The SyncML synchronization service comprises first initializing a synchronization session during which e.g. the database to be synchronized is selected. The SyncML client terminal MS synchronization application layer functions are provided by a synchronization client agent, which implements the SyncML protocol inter alia by sending a SyncML package (Client Modifications), which includes, in one or more SyncML messages, modifications made after the last synchronization session to the data that is the object of synchronization in the mobile station MS. The SyncML server S synchronization application layer functions are provided by a sync server agent, which controls synchronization, and a synchronization block (Sync Engine). The server usually waits for an initiative for synchronization from the SyncML client (MS). The server S synchronizes the data, i.e. analyses the changes made to the database and client terminal data, and harmonizes it (makes necessary modifications, replacements and deletions). After this, the SyncML server S sends the server modifications back to the SyncML client (MS). The example described above is simple, yet it illustrates the roles of the devices according to the SyncML standard. The SyncML client terminal (MS) is typically a mobile station (MS), a PC (Personal Computer), a laptop, or a PDA device. The SyncML server S is typically a network server or a PC.
The SyncML synchronization protocol operates in both wireless and wired networks and supports several transfer protocols. The SyncML synchronization protocol can be implemented, for example, on top of the HTTP protocol (Hyper Text Transfer Protocol), the WSP protocol (Wireless Session Protocol) of the WAP (Wireless Application Protocol) standard, the OBEX protocol used for cable links, such as the USB (Universal Serial Bus) or RS-232, or for short-range radio frequency (Bluetooth) links or infrared (IrDA) links, on top of a TCP/IP (Transport Control Protocol/Internet Protocol) stack, and also on top of an e-mail protocol (SMTP, Simple Mail Transfer Protocol). There are typically several different transmission media between the devices (MS, S) of a SyncML session, for instance a GSM network providing a wireless connection, and a local area network LAN. Also many transport layer protocols may be used to transfer SyncML messages. Different transmission media and the devices involved in the SyncML session may have different properties, e.g. varying data rates and packet sizes. The SyncML consists of end-to-end transmission of SyncML messages and it has to function even though a plurality of transport layer protocols is used. The devices are not typically aware of the message size they should use when sending synchronization messages to each other. If a device sends a large SyncML message, it is possible that the transport layer is not able to deliver it or a receiving device is not able to process it after it has arrived.